Smithsonian Institution
Bulletin of the Global Volcanism Network, v. 20, no. 11/12, November-December 1995
Cerro Negro (Nicaragua)  Vigorous eruption produces a new cone,
     dome, lava flows, and large ash plumes 

Cerro Negro
12.51 N, 86.70 W; summit elev. 675 m
All times are local (=3D GMT - 6 hours)

A significant eruption in November-December followed almost six
months of unrest and minor eruptive activity (Bulletin v. 20, nos.
5 & 9). During a crater visit on 13 November no precursors were
observed, and on 18 November only background seismicity was
recorded by the CNGN station (500 m E of the crater).

Early phase of activity, 19-22 November. Local residents first
noticed explosions about the time of the onset of 30 minutes of
mildly increasing seismicity detected by the CNGN station at 1145
on 19 November. Following a pause, seismicity continued to gain
strength. Increasing activity was reported that afternoon by
residents in Malpaisillo (~10 km N). Observations on the night of
19-20 November indicated mild Strombolian activity, with vertically
directed ejecta, that was gradually increasing in strength. A
Notice to Airmen (NOTAM) was issued the next day warning aviators
of the volcanic activity.

Eruption tremor amplitude increased continuously and saturated the
CNGN station (60 dB gain) at 0200 on the 21st. Tremor was detected
on short-period seismic stations within a 30 km radius (at San
Cristobal and Momotombo volcanoes, and near the city of Leon).
Energy release increased continuously and tremor could be felt over
1 km away, when sitting down, as a smooth rocking motion.

At 2000 on 21 November incandescent bombs were being thrown up to
300-400 m above the 1992 crater rim. Ash content was low compared
with the 1992 and May-August 1995 activity, and bombs were often
very large (meters across), which deformed and broke up in flight.
Because of near-vertical trajectories, few bombs fell outside the
crater. The new cone being built within the 1992 crater (figure 1)
had a steep (>45 degrees) basal scarp, 2-5 m high, followed by a
level bench and then a less steep slope (25 degrees) to its crater.
Ejecta pulses maintained a frequency of 20/minute, but the size and
duration of each pulse varied. From 0255 to 0310 on 22 November
ejecta heights were <150 m but ash content and degassing were much
higher, emitting dark clouds with each explosion. A thick, white
lower plume appeared to be escaping from a new lava dome in the
1992 crater, 50 m W of the new cone (figure 1). By 0500 the
eruption had regained previous intensity levels and exhibited
near-constant fire-fountain-like activity, bombs were larger, and
pulse frequency increased to 22/minute. The eruption continued at
this level for over 4 hours.

The new cone had almost reached the lip of the 1992 crater by 0700
on 22 November. At that time the lava dome emitted a small lava
flow, 2-5 m wide and 50 m long, that followed the edge of the new
cone towards the lowest part of the 1992 crater (figure 2). From
0930 to 1000 a series of explosions ejected material to the lower
slopes of the new cone. Sand to gravel size ash fell W of the cone,
but no large ejecta. Compared to the 1992 ejecta this material is
highly vesicular with millimeter-size vesicles; olivine, pyroxene,
and plagioclase are present, and some plagioclase crystals are 1 cm
long. That evening the new cone overgrew the N rim of the 1992
crater and material began spilling towards Cerro La Mula. From 1900
to 2300 a tongue of lava spilled over the N rim of the 1992 crater.
The front moved at <1 m/hour, but blocks constantly tumbled from
the front down to the base of the main cone.

Lava flows beyond the crater, 23 November. After 1400 on 23
November dark gray pulses observed from 25 km away formed a plume
that rose faster and higher than on previous days, attaining
several kilometers altitude. Observations were made from the
seismic station after 1500. During about 1515-1525 the plume became
less ash-rich, ejecta became less frequent, and strong degassing
pulses were heard. When regular pulses resumed, some bombs were
ejected laterally onto the flanks of the main cone. Periodic heavy
falls of 1-3 cm scoria were encountered by the scientists walking
under the plume 1.5 km from the cone. Red glow was visible at 1730
over Cerro La Mula, and there was a smell of burning vegetation,
suggesting an active lava flow. The lava tongue was observed at
1800 between Cerro La Mula and Cerro Negro (figure 2). Later named
the La Mula flow, it was ~20 m wide and 5 m thick, and advancing at
~2 m/hour.

At 1830 a 20-m-wide lava stream moved down the N flank through a
small breach at a rate of ~150 m/minute from the crater rim to the
base of the cone. A lava field spreading out from the base of the
cone had reached ~1 km from the crater by 2000, advancing 10-30
m/hour along two 300-m-wide fronts (figure 2). To the E of the flow
the volcano flank appeared to be bulging and was irregular with
large blocks jutting out that occasionally fell downslope,
revealing incandescent lava. It appeared to the scientists that a
slow-moving 20-m-thick blocky lava flow was moving to the crater
rim and collapsing down the flank; however, the shape of the flank
also suggested outward bulging. The blocky lava extended at least
200 m NE from the base of the cone.

Continuous and voluminous pulses at 2000 created a fountain that
sent bombs at least 600 m above the crater. Ash clouds accompanied
each pulse and occasional flames of burning gas reached 100-200 m
above the crater. This activity had decreased by 2045, and by 2115
pulses of bombs appeared only every 30 seconds, although continual
noise suggested smaller pulses.

Of the four GPS stations set up in the vicinity of the cone, by 23
November one had been destroyed by lava and another was too
dangerous to approach. Measurements at the remaining stations were
within the error of the equipment (2 cm at best). However, two
fresh fault scarps radial to the cone were observed on the W side
with 5 cm of displacement. Tremor energy increased continuously
until 1200 on 23 November, after which it maintained a constant

Continuing activity, 25-26 November. The eruption plume was again
clearly visible on 25 November from Managua as a diffuse gray
column turning horizontal at ~2,000 m. At 0900 distinct pulses of
dark gray ash rose from the crater and formed mushroom shapes
before drifting W and being incorporated into the plume; ashfall
was reported in Leon and Corinto. At times only massive bombs were
thrown out, while at others strong explosions sent up dense ash
clouds. Ash and highly vesicular scoria <5 cm in size fell
continuously on the W base of the cone. Pulses of ash and bombs had
a frequency of 20/minute, a characteristic periodicity in this
eruption. Pulses were strong enough to maintain a constant fountain
of bombs as high as 600 m, some of which were large and visibly red
5 km away; explosions were audible at this distance.

At 1100 on 25 November most bombs were still ejected vertically,
but a significant number were exiting at low angles and falling low
on the flanks. The new cone had grown to ~40 m across, and its top
was ~30-50 m below the 1992 crater summit. Bombs fell mostly on the
cone and rolled down to the base. The small breach where the 23
November lava flow exited was partly covered by a new blocky flow,
which appeared to come straight N from the new cone, though no exit
vent was visible. It may have been produced by accumulated, still
liquid ejecta beginning to flow outwards, as seen on 22 November.
The flow had advanced half way down the flank, covering another
blocky flow. The dome in the crater had grown to ~100 m wide and 40
m high. Blocks were continually spalling off the dome, which also
sustained a continuous rain of bombs from the new cone. Multiple
small lava tongues originated from the dome. The crater dome was
less pronounced on 26 November, and was blocky rather than spiny.
The new cone had grown ~10 m overnight.

The two flows moving N on the 23rd had reached ~1-1.5 km from the
volcano. The larger W lobe was ~400 m wide and 3-5 m thick at the
front with a small lobe extending down the gully below Cerro La
Mula, and another extending E into a depression in the old N lava
field. The E lobe had extended into forest at the E side of the old
N lava field. Over a three-hour period the flows advanced ~12 m. A
low ash-covered area with a small old cinder cone separated the
lobes. The sides of each flow were slowly (~1 m/hour) encroaching
on this and thickening. The thick lava lobes below the dome were
advancing, and many areas of the dome were glowing. The ~30-m-wide
La Mula lava flow had advanced W ~500 m down a small valley and was
moving at ~1 m/hour on 25 November; by 0600 on the 26th it had
stopped. By 0645 the other lava fronts had advanced 20-50 m since
the previous evening. The main W lobe had spread E and a large
block in the middle of the flow had moved ~100 m.

Seismic tremor levels remained high through 26 November. Tremor was
continuous and distinctly felt up to 1.5 km from the cone.

Satellite observations of the ash plume. Visible satellite imagery
on 25 November indicated a possible low-level ash cloud at 1245
(figure 3). The height of the plume was estimated at 4,500 m
altitude and was moving SW at ~30 km/hour. Another small low-level
plume was seen on imagery at 0815 the next day at an estimated
2,750 m altitude and moving WSW at ~35 km/hour. Explosive activity
increased on 1 December, when visible imagery at 1230 revealed a
plume 18 km wide extending ~320 km W; it was estimated to be
between 3,000 and 6,000 m altitude. By 0900 on 2 December, the
plume extended at least 640 km W and was below 4,000 m.

End of the eruption, early December. Explosive and effusive
activity ended on 6 December. However, a lava flow was still moving
N on 8 December. Isopach maps of the ashfall through 2 December
(figure 4) were constructed by Markus Kesseler based on 85 GPS
control points (precision +- 30 m). The 0.1 cm isopach encloses an
area of ~200 km^2. An estimated 12,000 people were affected by this
eruption, about 6,000 of whom had been evacuated from 15 rural
communities. Farmland was significantly damaged by ashfall and lava
flows during the harvesting season; most of those affected were
farmers and their families.

Information Contacts: Wilfried Strauch, Virginia Tenorio, Rolf
Schick, Helman Taleno, Leonel Urbina, Cristian Lugo, and Pedro
Perez, Instituto Nicaraguense de Estudios Territorales, Managua,
Nicaragua (Email:; Benjamin van Wyk de Vries, The
Open University, Milton Keynes, United Kingdom (Email; Markus Kesseler, Dept. of
Mineralogy, Universite de Geneve, 13 rue des Maraichers, 1211
Geneve 4, Switzerland (Email:; Michael
Conway and Brittain E. Hill, Center for Nuclear Waste Regulatory
Analyses, Southwest Research Institute, 6220 Culebra Rd., San
Antonio, TX 78238 USA (Email:;; Jim Lynch, Synoptic Analysis Branch, NOAA/NESDIS,
Room 401, 5200 Auth Rd., Camp Springs, MD 20746 USA; Department of
Humanitarian Affairs, United Nations, Palais des Nations, 1211
Geneva 10, Switzerland.

Figure 1. Sketch of the crater at Cerro Negro, 0700 on 22 November
1995. Drawn from photographs taken by Pedro Perez; courtesy of

Figure 2. Sketch map of Cerro Negro showing active lava flows, 2000
on 23 November 1995. Drawn by B. Van Wyk de Vries; courtesy of

Figure 3. Map showing ash plumes from Cerro Negro detected on
visible satellite imagery on 25-26 November, and 1-2 December 1995.
Courtesy of the Synoptic Analysis Branch, NOAA/NESDIS.

Figure 4. Isopach maps of ashfall from Cerro Negro, 19 November-2
December 1995. Isopachs within the 5.0 cm limit are at 10-cm
intervals, up to 50 cm closest to the crater. The 2-5 June isopachs
(Bulletin v. 20, no. 9) are shown for comparison. Courtesy of
Markus Kesseler; base map courtesy of Brittain Hill.